Experiments are designed in chicks to obtain separation of thymus-derived (T) and bursa-derived (B2) cells and to characterize functional capacities of B cell subpopulations, through the combined use of chromosome markers, cell separation techniques and tissue culture methods. Related objectives include determination of mechanisms and extent of cellular interactions between T cells, B cells and macrophages in response to specific antigens and nonspecific stimulants. In addition, chromosome marker techniques are employed in experiments designed to define the various sites of origin of B cell precursors in chick embryos.